FIG. 4 is a cross-sectional view of the shoe press belt for papermaking, and FIG. 5 is a cross-sectional view of a shoe press apparatus for papermaking.
As shown in FIG. 5, a shoe press for papermaking comprises a shoe press mechanism including a looped shoe press belt 10 interposed between a press roll 1 and a shoe 5. In this mechanism, the press roll 1 and the shoe 5 provide a pressing region therebetween through which a feed felt 3 and a wet paper web 4 are caused to pass to dehydrate the wet paper web 4.
As shown in FIG. 4, the shoe press belt 10 comprises an outer circumferential polyurethane layer 21 and an inner circumferential polyurethane layer 22 which are disposed respectively on both surfaces of a reinforcing fiber base 6, which is sealed (embedded) in the polyurethane layers.
The outer circumferential polyurethane layer 21, which is held against the press roll, has a number of concave grooves 24 defined in its surface. Water, which is squeezed from the wet paper web 4 when it is pressed in the pressing region, is held in the concave grooves 24, and then brought out of the pressing region as the shoe press belt 10 rotates.
Ridges 25 formed on the outer circumferential polyurethane layer 21, which is held against the press roll, are required to have their mechanical properties improved. The mechanical properties include crack resistance, flexural fatigue resistance and wear resistance, etc. against vertical pressing forces applied by the press roll 1 and against wear and flexural fatigue of the shoe press belt in the pressing region.
For the above reasons, polyurethane having an excellent crack resistance is widely used as the resin material of the outer circumferential polyurethane layer 21 of the shoe press belt 10.
Shoe press belts, for example, comprise a reinforcing fiber base and a polyurethane layer, which are integrally combined with each other. The polyurethane layer comprises an outer circumferential layer and an inner circumferential layer. The reinforcing fiber base is embedded in the polyurethane layer.
JP, A, 2002-146694 and JP, A, 2005-120571 disclose shoe press belts made of polyurethane.
The outer circumferential layers of these shoe press belts are made of polyurethane having a “JIS A hardness ” ranging from 89 to 94. The polyurethane comprises a mixture of a urethane prepolymer (HIPRENE L: trade name, manufactured by Mitsui Chemicals, Inc.) and a curing agent containing dimethylthiotoluenediamine. The equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value in the range of 1<H/NCO<1.15.
The mixed composition of the urethane prepolymer and the curing agent is cured into the polyurethane. The urethane prepolymer is produced by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG). The urethane prepolymer has a terminal isocyanate group.
The inner circumferential layers of the papermaking belts are made of polyurethane comprising a mixture of a urethane prepolymer (manufactured by Mitsui Chemicals, Inc.) and a mixed curing agent. The urethane prepolymer and the mixed curing agent are mixed such that the equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value in the range of 0.85≦H/NCO<1.
The urethane prepolymer is produced by reacting 4,4′-methylene bis(phenyl isocyanate) (MDI) with polytetramethylene glycol (PTMG). The urethane prepolymer has a terminal isocyanate group. The mixed curing agent contains 65 parts of dimethylthiotoluenediamine and 35 parts of polytetramethylene glycol (PTMG).
The mixed composition of the urethane prepolymer and the curing agent is cured into the polyurethane. The shoe press belt is formed of these polyurethanes.
A papermaking shoe press belt also disclosed in JP, A, 2005-307421 comprises an integral structure of a reinforcing fiber base and a polyurethane layer. The polyurethane layer of the belt comprises an outer circumferential layer and an inner circumferential layer. The reinforcing fiber base is embedded in the polyurethane layer.
The polyurethane, of the outer circumferential layer and the inner circumferential layer of the belt, comprises a mixture of a urethane prepolymer (HIPRENE L: trade name, manufactured by Mitsui Chemicals, Inc.) and a curing agent containing dimethylthiotoluenediamine. The urethane prepolymer and the curing agent are mixed such that the equivalent ratio (H/NCO) of an active hydrogen group (H) of the curing agent and an isocyanate group (NCO) of the urethane prepolymer has a value of 0.97.
The urethane prepolymer is produced by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG), and has a terminal isocyanate group.
The mixed composition of the urethane prepolymer and the curing agent is cured into the polyurethane which has a “JIS A hardness” ranging from 94 to 95.
A shoe press belt disclosed in JP, A, 2006-144139 comprises an integral structure of a reinforcing fiber base and a polyurethane layer. The reinforcing fiber base is embedded in the polyurethane layer.
The polyurethane of the shoe press belt is made of a urethane prepolymer and a curing agent which are mixed such that the equivalent ratio (H/NCO) is in the range of 0.9≦H/NCO≦1.10.
The urethane prepolymer contains unreactive polydimethylsiloxane in a liquid form. The urethane prepolymer is produced by reacting tolylene diisocyanate (TDI) with polytetramethylene glycol (PTMG), and has a terminal isocyanate group.
The curing agent is selected from a group consisting of dimethylthiotoluenediamine (trade name: ETHACURE300) and 4,4-methylene bis-(2-chloroaniline) {MOCA}.
The mixed composition of the urethane prepolymer and the curing agent is cured into the polyurethane which has a “JIS A hardness” ranging from 93 to 96.
JP, A, 2006-144139 also proposes another shoe press belt. The proposed shoe press belt is made of polyurethane which is a mixture of polyurethane, having a “JIS A hardness” ranging from 90 to 93 and containing unreactive polydimethylsiloxane in a liquid form, and polyurethane having a “JIS A hardness” of 98 and free of unreactive polydimethylsiloxane in a liquid form. The mixture is mixed with a curing agent of dimethylthiotoluenediamine at an equivalent ratio in the range of 0.9≦H/NCO≦1.10.
The mixed composition of the urethane prepolymer and the curing agent is cured so that the shoe press belt which has a “JIS A hardness” ranging from 90 to 93 is formed.
Patent Document 1: JP, A, 2002-146694
Patent Document 2: JP, A, 2005-120571
Patent Document 3: JP, A, 2005-307421
Patent Document 4: JP, A, 2006-144139
The embodiments of JP, A, 2002-146694, JP, A, 2005-120571, JP, A, 2005-307421 and JP, A, 2006-144139 disclose shoe press belts.
The shoe press belts are measured by an inspecting apparatus. For measurement, the opposite ends of a test piece of the belt are gripped by clamp hands. The cramp hands are reciprocally movable horizontally in a ganged fashion. The test piece has an evaluation surface facing a rotating roll, and the press shoe moves toward the rotating roll to press the test piece for measuring crack resistance thereof.
While the test piece was subjected to a tensile force of 3 kg/cm and a pressure of 36 kg/cm2 by the inspecting apparatus, the clamp hands were reciprocally moved at a speed of 40 cm/sec., and the number of times that the clamp hands were reciprocally moved was measured until the test piece was cracked. As a result, it was found that no crack developed in the test piece after the clamp hands were reciprocally moved 1,000,000 times.
In recent years, the shoe press belts have been used in highly severe environments as the operating speed has increased to meet demands for higher paper productivity growth, the shoe press belts have had an increased width of about 10 m, and the pressure applied in the pressing region has become higher. Therefore, the various properties of the shoe press belts need to be improved further.
The present invention has been made to solve the above problems. It is an object of the present invention to provide a shoe press belt for papermaking which has excellent mechanical properties in crack resistance, flexural fatigue resistance, wear resistance, etc.